Understanding the interaction between gene expression and electrical activity in the nervous system is an essential problem in neuroscience that has broad impacts on such areas as synaptic plasticity and neurodevelopment. A useful model system to study this interaction is the brain's biological clock, the suprachiasmatic nucleus (SCN). The SCN generates endogenous molecular and electrical circadian rhythms that together regulate a multitude of physiological processes. Disrupted circadian rhythms have the potential to influence a range of human health conditions such as sleep and metabolic disorders and neuropsychiatric illnesses such as autism and depression. To understand how the dysregulation of circadian clock components may lead to disease, it is first necessary to understand how they work in a healthy brain. A key unsolved question in circadian neurobiology is how the SCN's molecular and electrical rhythms interact to form a coherent pacemaker. SCN neurons are autonomous oscillators that possess daily molecular transcriptional- translational feedback loops involving the core clock genes Per1/2, Cry1/2, Bmal1, and Clock. These neurons can spontaneously fire action potentials, and, importantly, can modulate their firing rates so that they fire quickly during the day and slowly at night. So far, there has only been indirect evidence connecting the molecular feedback loop and SCN electrical activity. This research plan proposes a unique strategy that combines electrophysiology, real-time clock gene expression imaging, and the optogenetic manipulation of firing rate to elucidate the link between gene expression and firing rate rhythms in the SCN. Specifically, electrophysiological recording will be combined with real-time clock gene expression imaging in a novel transgenic mouse line (Per1:GFP mice) in which fluorescence is a readout of transcriptional activation of the core clock gene Per1 to determine the inherent phase relationship between firing rate and Per1 promoter activity. These techniques will also be used in animals in which Per1 is knocked out or overexpressed to investigate whether Per1 itself is a functional link between the molecular clock and electrical activity rhythms. Conversely, the influence of firing rate rhythms on circadian gene expression will be examined by using a combination of SCN-specific optogenetic manipulation of firing rate and time-lapse confocal imaging of clock gene expression in Per1: GFP mice expressing SCN-specific light-gated ion channels. These techniques will be used to determine how increasing, decreasing, or eliminating firing rate rhythms alter gene expression rhythms in the SCN. This research plan will ultimately help elucidate the relationship between SCN gene expression and electrical activity rhythms, which will improve our understanding of the interplay of essential circadian clock components whose dysfunction can negatively impact human health.